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Magnitude 8.6 earthquake sparks tsunami

A major earthquake has struck off the west coast of Sumatra in Indonesia.

Initial automated assessments rated the quake as a magnitude 8.9, but it is now thought to have been an 8.6. It struck at 0838 UTC this morning, approximately 400 kilometres south-west of the Sumatran mainland, at a depth of 33 kilometres.

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The latest quake is in the same region. “A lot of the damage last time was due to the tsunami,” says Julian Bukits of the British Geological Survey.

According to Amy Vaughan of the US Geological Survey, the event seems to have been a strike-slip quake in which two sections of a tectonic plate move horizontally, grinding alongside each other. The San Andreas fault in California moves in a similar way. Off Indonesia’s coast, the Australian plate is being subducted beneath the Eurasian plate. However the quake appears to have occurred on the edge of the subduction zone, along two faults, rather than at the trench itself.

By contrast the 2004 quake was a thrust earthquake in which one tectonic plate was violently forced under another. Such quakes involve a lot of vertical motion, triggering particularly large tsunamis.

Because today’s quake was horizontal, it is likely that any tsunami will be smaller than the devastating wave that struck in 2004. “I remain hopeful that this is not going to be so devastating,” Vaughan says.

A magnitude 7.2 quake struck nearby on 10 January this year, and may have disturbed the region’s faults, preparing the ground for a major earthquake.